It is well-known that the processes of chemical vapor deposition ("CVD"), physical vapor deposition ("PVD") and plasma etch occur within a reactor. One exemplary prior art reactor is described in U.S. Pat. No. 5,273,588, entitled "Semiconductor wafer processing reactor apparatus comprising contoured electrode gas directing means." Typically, the reactor of the prior art has an outer chamber which surrounds a pair of electrodes driven by a radiofrequency ("RF") source. The electrodes and source, in combination, generate a discharge between the electrodes to ionize reactive gases therein. These ionized gases form a "plasma" which deposits film onto, or etches film off of, surfaces in contact with the plasma.
In plasma CVD, for example, a workpiece, e.g., a semiconductor wafer, is clamped to one of the electrodes so that selected films can be deposited onto the workpiece's surface(s) exposed to the plasma between the electrodes. Successive exposures to differing plasmas can create desirable semiconductor films on the surface, such as a bilayer of titanium and titanium-nitride. Similarly, in plasma etch, coatings or films can be removed selectively when exposed to plasmas formed by appropriate etch gases, e.g., carbon tetrafluoride.
Because it is often desirable to heat the workpiece during the CVD or PVD process, one of the electrodes (usually the one that the wafer rests upon) also generally functions as a heated carrier or "susceptor". Accordingly, the heated electrode must be monitored and controlled to maintain electrode temperatures during film deposition or etch. Such control is particularly challenging in order to attain the desired accuracies of +/- two degrees, or less, for electrodes of up to seven hundred degrees Centigrade.
In the prior art, the heated electrode is generally formed by implanting a heating cable within the electrode. FIG. 1 illustrates one prior art reactor 10 configured with a heated metal susceptor 12. A wafer 14 is illustratively shown connected to the heated susceptor 12 and between the gap 16 formed by the susceptor 12 and the second electrode 18. The drive electronics, including the RF generator, are not shown for clarity of illustration.
The heated susceptor 12 forms a hollow interior 12a and includes a backplate 22 and a cable heater 20, each described in more detail in connection with FIGS. 1A, 1B and 1C. Because the chamber walls 11 of the reactor 10 enclose the electrodes 12, 18 within a low evacuated pressure (denoted as "[VACUUM]"), the susceptor 12 connects to a shaft 24 that extends out of reactor 10 and that couples ambient pressure (denoted as "[ATM]") to the interior 12a. Metal seals 26 properly seal the shaft 24 to the susceptor 12. A metal bellows 28 connects the walls 11 to the shaft 24 in order to provide for vertical movement of the shaft 24 and susceptor 12 along direction 31. The seals 30 connect between the walls 11 and the bellows 28 so as to maintain vacuum within the reactor 10. The cable heater 20 connects to drive electronics external to the reactor 10 and through the shaft 24 by way of cable leads 32.
FIG. 1A shows a cross-sectional and cut-away side view of the heated susceptor 12 of FIG. 1. The susceptor casing 12b connects with the backplate 22 and forms a series of conduits 12c for the cable heater 20. More particularly, the backplate 22--shown in a top view in FIG. 1B--and susceptor casing 12b are machined so as to provide a conduit 12c for the cable heater 20. As illustrated, the cable heater 20 is typically coiled and formed into a desired pattern matching the conduit 12c machined into the backplate 22 and casing 12b. The cable heater 20 connects to the cable leads 32 (shown only in FIG. 1) via through-holes 22a. A typical, straight cable heater 20' is shown as a section view in FIG. 1C. As illustrated, the cable heater 20' includes an outer metal casing 20a, a coiled active wire 20b, and an insulating powder 20c, e.g., MgO, that packs the wire 20b within the casing 20a.
Those skilled in the art are familiar with cable heaters and electrodes such as the cable 20 and susceptor 12. In operation, the wire 20b within cable 20' is heated when connected to electrical energy. The electrically insulating powder 20c is thermally conductive so that the thermal energy from the wire 20b transmits to the casing 20a, which radiates heat to the backplate 22 and susceptor 12. Note that, unlike the illustrated susceptor 12 and cable heater 20, FIG. 1A, operational susceptors have filaments that are coiled tightly within the susceptor so as to ensure uniform heating of the workpiece, e.g., the wafer 14 of FIG. 1. The cable heater 20 and susceptor design shown in FIGS. 1-1C are shown with reduced complexity for purposes of illustration.
The susceptor 12 of FIGS. 1 and 1A is typically constructed from stainless steel or stainless-covered copper. It is limited, in the prior art, to metal for reasons of vacuum sealability. Even the seals are typically made from metal. The relatively thick construction of the susceptor 12 is thus costly and leads to a high thermal mass, increasing the time required to heat the susceptor. It further results in poor thermal conductivity and leads to certain non-desirable thermal non-uniformities.
Other problems exist within the design of FIG. 1. First, major disassembly of the system 10 and susceptor 12 is required to replace the heater 20 in the event of failure. Accordingly, the repair time is long; and the associated cost to the production line is relatively high. Secondly, the feeds to the susceptor 20 are forced to come through a hollow shaft 24. Such a design is very constricting and leaves little room for even minor use or constructional modifications. Finally, because of the poor conductivity of susceptor 12, the heater 20 is more likely to fail because it must operate at higher temperatures in order to maintain the susceptor's surface temperature at the desired level.
FIG. 2 illustrates another prior art reactor 40 with a different heated susceptor configuration 42. A cable heater 44, like the one described above, is wound within the machined susceptor top plate 42a and against the backplate 42b. The susceptor 42 is suspended within the reactor 40 by way of the mount 50. The susceptor 42 is held within a vacuum, so the cable heater 44 is connected to drive electronics via hermetic electrical terminals 46, known to those skilled in the art. The terminals 46 connect to lead wires 47 which pass though the reactor wall 40a by way of vacuum electrical feed-through 48. For purposes of clarity, other reactor components are not illustrated, such as the second electrode and supporting electronics.
FIG. 3 illustrates another prior art reactor 60 with a different heated susceptor configuration 62. The susceptor 62 includes a thermofoil heater 62a--described in greater detail below--that mounts between a susceptor casing 62b and a susceptor backplate 62c. The susceptor 62 is suspended within the reactor 60 by way of the mount 66. The susceptor 62 is held within a vacuum, so the thermofoil heater 62a is connected to heater lead wires 68 which pass though the reactor wall 60a by way of vacuum electrical feed-through 70. As in FIG. 2, other reactor components are not illustrated for purposes of clarity, such as the second electrode and supporting electronics.
Heating susceptors configured as in FIGS. 2 and 3 are particularly subject to failure because of any of the following conditions: contamination, corrosion, over-heating, and dielectric breakdown. With regard to FIG. 2, for example, the heater elements--i.e., the cable heater 44, and the plates 42a, 42b--are held within VACUUM and are exposed directly, or even indirectly, to processing environments which can be especially harsh, including reactive RF plasmas and corrosive gases. The heater 44 is further exposed, at times, to aggressive CVD or cleaning chemistries that can attack the heater 44. After prolonged exposure, the heater 44 becomes seriously contaminated with unwanted residue. Further, because the cable 44, terminals 46 and lead wires 47 are directly exposed to the environment within reactor 40, over time the dielectrics associated with these elements become metalized by CVD processes, or become corroded by process chemistries, and are thus subject to failure. The heater 44 must also operate at high temperatures in order to overcome the thermal transfer inefficiency of the evacuated environment within the chamber and to heat the susceptor 42. It is thus more likely to fail when forced to operate at higher temperatures.
FIG. 3A shows a cross-sectional view of the thermofoil heater 62a, which includes two boron nitride discs 70a, 70b as electrical insulators about an Inconel trace element 70c. A boric oxide adhesive 71 binds the two discs together. Note that the trace element 70c is in reality wound tightly within the susceptor 62 in a pattern machined into the discs 70a, 70b.
In addition to certain of the problems described above in connection with FIG. 2, the reactor 60 has other problems. The thermofoil heater 62a and lead wires 68 are directly exposed to the reactor 60 environment, and are prone to failure due to the corrosion and/or metalization. The Inconel trace 70c can also evaporate in a vacuum, creating contamination by re-depositing on cooler surfaces or causing dielectric breakdown of the insulating boron nitride by re-deposition on the insulators. Other elements, e.g., leadwires and insulating discs 70a, 70b, are also in the evacuated pressures and can cause additional outgassing and contamination of the process environment. The evaporation of the Inconel trace 70c further reduces the cross-sectional area of the trace, thereby increasing resistance and creating "hot" spots: hot spots can cause thermal non-uniformities and accelerate the failure of the trace 70c which can eventually cause an electrical "open" or short-circuit of the heater 62a. CVD processes also can metalize the discs 70a, 70b and leadwire insulation, leading to heater current leakage and eventual short-circuiting. Finally, the nitride discs 70a, 70b tend to create particulates, and thus unwanted contamination within the reactor 60.
Accordingly, there is the need to prolong the life of the heated electrode and associated heating elements; and to make reactor operation continually efficient over time. It is thus one object of the invention to provide a heating susceptor which reduces, or eliminates, the problems associated with prior art heating electrodes. Other objects of the invention will be apparent from the description which follows.